The present invention relates to a golf ball capable of exhibiting a uniform flying distance of the golf ball by improving an aerodynamic isotropy of the golf ball.
A large number of dimples are arranged on the outer surface of a golf ball for increasing the flying performance of the golf ball. The flying performance of a golf ball, however, varies depending on the rotational direction (the latitudinal direction, longitudinal direction, or the intermediate direction therebetween) of the golf ball, which differs depending on a ball hitting point. The variation in rotational direction of the golf ball depending on a ball hitting point is mainly due to the distribution of the dimples arranged on the surface of the ball. From this viewpoint, various methods have been proposed to reduce a variation in flying performance of a golf ball by enhancing the uniformity in arrangement of dimples on the surface of the golf ball.
For example, a golf ball has been proposed in Japanese Patent Publication No. Hei 6-7875, in which the shapes of dimples are adjusted so that an effective total volume of the dimples of the golf ball which is hit at its north or south pole portion and is thereby rotated around the rotational axis corresponding to the equatorial line is nearly equal to an effective total volume of the dimples of the golf ball which is hit at its seam portion and is thereby rotated around the rotational axis corresponding to the line connecting the north and south poles to each other.
Another golf ball has been proposed in Japanese Patent No. 2910707, in which dimples are designed so that at least one of a specific volume symmetry index Vi, a specific area symmetry index Si, a specific edge length symmetry index Li, and a specific arrangement symmetry index Ni is more than 1, to adjust the dimple arrangement so that the dimple effect does not differ between when the golf ball is hit at its seam portion and when it is hit at its north or south pole portion, thereby obtaining a stable trajectory of the golf ball.
Each of the above-described prior art golf balls, however, has a problem. Namely, such a golf ball is intended to allow the aerodynamic characteristic of the golf ball rotated around one limited rotational axis (for example, a pole axis) to correspond to the aerodynamic characteristic of the golf ball rotated around another limited rotational axis (for example, seam axis); however, in actual golf play, since the golf ball is hit at any point, it may be rotated around a random rotational axis, and accordingly, the prior art golf ball fails to have a uniform aerodynamic characteristic for rotation of the ball around any rotational axis, that is, fails to exhibit a uniform flying performance.
On the other hand, a golf ball has been conventionally needed to have a great circle (parting line) with no dimple intersecting therewith for forming the golf ball by molding. Such a parting line has been generally prepared by disordering part of a regular polyhedron arrangement or preparing a pseudo-great circle; however, in this case, since the dimple arrangement is distorted or a land with no dimple is continuously formed, there arises a problem that the uniformity of the aerodynamic characteristic of the golf ball is degraded.
By the way, dimples have been arranged, in general, on the spherical surface of a golf ball by a method of assuming the spherical surface of the golf ball as a spherical regular polyhedron and using one plane of the spherical regular polyhedron as a dimple arrangement unit. Such a dimple arrangement method is effective to facilitate the dimple design and to ensure a certain aerodynamic isotropy of the golf ball.
In this method, however, if the number of spherical unit planes constituting the spherical regular polyhedron is small, there may occur a large difference in aerodynamic characteristic between when the golf ball is rotated around a rotational axis along the boundary between the units and when it is rotated around a rotational axis along a line offset therefrom. That is to say, in this case, since the rotational axis of the golf ball differs depending on a ball hitting point because of the fact that the number of spherical unit planes constituting the spherical polyhedron is small, there arises a problem that the aerodynamic characteristic due to the regularity of the dimple arrangement varies and thereby the flying distance of the golf ball also varies.
For example, in the above method, if the number of the spherical unit planes of the spherical regular polyhedron constituting the spherical surface of the golf ball is two, that is, if the golf ball is assumed by combination of two semi-spheres, the difference in aerodynamic characteristic between when the golf ball is rotated along the equatorial direction and when it is rotated along the longitudinal direction becomes large to such an extent as to be acknowledged by a player in the actual test. Additionally, such a difference in aerodynamic characteristic has come to be numerically measured by a test using a hitting machine (M/C).
Various methods have been proposed to improve the above-described difference in aerodynamic characteristic of a golf ball depending on a ball hitting point; however, in either of these methods, it is not intended to basically improve the difference in aerodynamic characteristic of a golf ball but to partially correct the difference in aerodynamic characteristic of the golf ball while keeping the basic dimple arrangement unit, and thereby it fails to significantly improve the aerodynamic isotropy of the golf ball.
An object of the present invention is to provide a golf ball capable of exhibiting a uniform aerodynamic characteristic even when the ball is rotated along any rotational axis, thereby exhibiting a uniform flying performance, and to provide a method of evaluating a golf ball.
To achieve the above object, according to one aspect of the present invention, there is provided a golf ball including: a large number of two types or more dimples different in diameter and/or depth, which are arranged on the golf ball is in a range of 2 or less, the EAstdev value being expressed by the following equations:             EA      ⁡              (                  α          j                )              =                  ∑                  i          =          1                N            ⁢              [                  S          ⁢                      xe2x80x83                    ⁢          i          xc3x97          sin          ⁢                      xe2x80x83                    ⁢                      (                          θ              i                        )                          ]                  EAstdev    =                                        N            ⁢                                          ∑                                  j                  =                  1                                N                            ⁢                                                [                                      EA                    ⁡                                          (                                                                        α                          ⁢                                                      xe2x80x83                                                                          j                                            )                                                        ]                                2                                              -                                    ∑                              j                =                1                            N                        ⁢                                          [                                  EA                  ⁡                                      (                                          α                      j                                        )                                                  ]                            2                                                N          ⁢                      xe2x80x83                    ⁢                      (                          N              -              1                        )                              
wherein N is the total number of dimples; each of suffixes xe2x80x9cixe2x80x9d and xe2x80x9cjxe2x80x9d is a dimple ID number; xcex1j is a rotational axis of a golf ball determined by a straight line connecting the center of the dimple having an ID number xe2x80x9cjxe2x80x9d to the center of gravity of the golf ball; EA (xcex1j) is an aerodynamic index of a dimple arrangement around the rotational axis xcex1j; xcex8i is an angle formed between the rotational axis xcex1j of the golf ball and a straight line connecting the center of the dimple having the ID number xe2x80x9cixe2x80x9d to the center of gravity of the golf ball; and Si is an area of a planar shape formed by a peripheral edge of the dimple having the ID number xe2x80x9cixe2x80x9d.
According to another aspect of the present invention, there is provided a method of evaluating a golf ball having on its surface a large number of two types or more dimples different in diameter and/or depth, including the steps of: calculating an EAstdev value of the golf ball; and estimating, if the EAstdev value is 2 or less, that the uniformity of a dimple arrangement of the golf ball is high and thereby an aerodynamic isotropy of the golf ball is good, wherein the EAstdev value is expressed by the following equations:             EA      ⁡              (                  α          j                )              =                  ∑                  i          =          1                N            ⁢              [                  S          ⁢                      xe2x80x83                    ⁢          i          xc3x97          sin          ⁢                      xe2x80x83                    ⁢                      (                          θ              i                        )                          ]                  EAstdev    =                                        N            ⁢                                          ∑                                  j                  =                  1                                N                            ⁢                                                [                                      EA                    ⁡                                          (                                                                        α                          ⁢                                                      xe2x80x83                                                                          j                                            )                                                        ]                                2                                              -                                    ∑                              j                =                1                            N                        ⁢                                          [                                  EA                  ⁡                                      (                                          α                      j                                        )                                                  ]                            2                                                N          ⁢                      xe2x80x83                    ⁢                      (                          N              -              1                        )                              
where N is the total number of dimples; each of suffixes xe2x80x9cixe2x80x9d and xe2x80x9cjxe2x80x9d is a dimple ID number; xcex1j is a rotational axis of a golf ball determined by a straight line connecting the center of the dimple having an ID number xe2x80x9cjxe2x80x9d to the center of gravity of the golf ball; EA (xcex1j) is an aerodynamic index of a dimple arrangement around the rotational axis xcex1j; xcex8i is an angle formed between the rotational axis aj of the golf ball and a straight line connecting the center of the dimple having the ID number xe2x80x9cixe2x80x9d to the center of gravity of the golf ball; and Si is an area of a planar shape formed by a peripheral edge of the dimple having the ID number xe2x80x9cixe2x80x9d.
With this configuration, the EAstdev value as the index indicating the uniformity of a dimple arrangement of a golf ball having on its surface a large number of two types or more dimples different in diameter and/or depth is in a range of 2 or less. Accordingly, unlike the prior art golf ball, in which the aerodynamic characteristic of the golf ball rotated around one limited rotational axis (for example, a pole axis) corresponds to the aerodynamic characteristic of the golf ball rotated around another limited rotational axis (for example, seam axis), the aerodynamic isotropy of the golf ball of the present invention can be kept constant even when the ball is rotated around any rotational axis, with a result that the golf ball of the present invention can exhibit a uniform flying performance allowing a golfer to certainly carry the golf ball toward a target location.